Phalenopsis with weird spotting under leaves

[sweta]

Hello all,

I bought this phal from trader joes about 1.5 years ago then repotted it after blooms fell off. Since then it lost all of its roots except 1 shriveled root and hasnt grown even one single new root. It grew 1 new leaf last year and recently started giving out 2 secondary spikes. I noticed weird reddish brown spotting on the stem and under the leaves and I'm not sure what it is. And wondering if I should be isolating the plant. I apply fertilizer once every 2 weeks and temperatures in our house are between 67F. I would appreciate if you there are any suggestions on how to treat the plant if its a disease or should i just toss out the plant? I'm worried if this could spread to other healthy plants